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modbus_new_rtu - create a libmodbus context for RTU


modbus_t *modbus_new_rtu(const char *device, int baud, char parity, int data_bit, int stop_bit);


The modbus_new_rtu() function shall allocate and initialize a modbus_t structure to communicate in RTU mode on a serial line.

The device argument specifies the name of the serial port handled by the OS, eg. "/dev/ttyS0" or "/dev/ttyUSB0". On Windows, it's necessary to prepend COM name with "\.\" for COM number greater than 9, eg. "\\.\COM10". See for details

The baud argument specifies the baud rate of the communication, eg. 9600, 19200, 57600, 115200, etc.

The parity argument can have one of the following values:

  • N for none
  • E for even
  • O for odd

The data_bits argument specifies the number of bits of data, the allowed values are 5, 6, 7 and 8.

The stop_bits argument specifies the bits of stop, the allowed values are 1 and 2.

Once the modbus_t structure is initialized, you can connect to the serial bus with modbus_connect.

In RTU, your program can act as server or client:

  • server is called slave in Modbus terminology, your program will expose data to the network by processing and answering the requests of one of several clients. It up to you to define the slave ID of your service with modbus_set_slave, this ID should be used by the client to communicate with your program.

  • client is called master in Modbus terminology, your program will send requests to servers to read or write data from them. Before issuing the requests, you should define the slave ID of the remote device with modbus_set_slave. The slave ID is not an argument of the read/write functions because it's very frequent to talk with only one server so you can set it once and for all. The slave ID it not used in TCP communications so this way the API is common to both.

Return value

The function shall return a pointer to a modbus_t structure if successful. Otherwise it shall return NULL and set errno to one of the values defined below.


  • EINVAL, an invalid argument was given.
  • ENOMEM, out of memory. Possibly, the application hits its memory limit and/or whole system is running out of memory.


In this example, the program will open a serial communication on USB. All subsequent calls such as read or write of registers will be sent on the wire and the request will be visible to all connected devices. According to the Modbus protocol, only the master associated to slave ID 10 will process and answer your requests.

const int REMOTE_ID = 10;
modbus_t *ctx;
uint16_t tab_reg[10];

ctx = modbus_new_rtu("/dev/ttyUSB0", 115200, 'N', 8, 1);
if (ctx == NULL) {
    fprintf(stderr, "Unable to create the libmodbus context\n");
    return -1;

if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    return -1;

modbus_set_slave(ctx, REMOTE_ID);

// Read 2 registers from address 0 of server ID 10.
modbus_read_registers(ctx, 0, 2, tab_reg);

See also